Recent years have seen development of crystalline-silicon-based semiconductor substrates including photodiodes and rapid implementation of finer pixels for charge coupled device (CCD) solid-state imaging devices or metal oxide semiconductor (MOS) solid-state imaging devices respectively including CCDs or MOSes as scanning circuits. A pixel size of 3 μm around the year 2000 became smaller than or equal to 2 μm in 2007. The solid-state imaging devices to come to the market in 2010 will include pixels of 1.4 μm. If the pixel size becomes finer at this pace, the resulting pixel size is expected to be as small as 1 μm or smaller within several years.
In order to achieve the pixel size of 1 μm or smaller, however, the inventors of the present invention have found out that there are two problems to be solved: a first problem due to a small optical absorption coefficient of crystalline silicon and a second problem due to amount of signals to be used. Described hereinafter are the details of the first problem. An optical absorption coefficient of crystalline silicon depends on light wavelength. Crystalline silicon of approximately 3.5 μm in thickness is required to almost completely absorb and photoelectrically-convert green light of approximately 550 nm in wavelength which determines the sensitivity of a solid-state imaging device. Hence, a photodiode, which is formed in a semiconductor substrate and operates as a photoelectric converting unit, needs to be approximately 3.5 μm in depth. When the size of a flat pixel is 1 μm, it is very difficult to form a photodiode of approximately 3.5 μm in depth. If the depth of a formed diode is approximately 3.5 μm, a highly possible problem to occur would be that oblique incident light on the photodiode could enter another photodiode whose pixel is adjacent to the light-receiving photodiode. The oblique incident light on the adjacent pixel causes color mixture (cross talk), which develops a serous problem for color solid-state imaging devices. Photodiodes could be formed thinner in order to prevent the color mixture; however, a photodiode thinner than the above ones could deteriorate the optical absorption efficiency of green light, leading to deterioration in sensitivity of the image sensor. A finer pixel becomes smaller in its pixel size, followed by a decrease in sensitivity per pixel. The decreasing optical absorption efficiency in addition to the decreasing sensitivity is detrimental to color solid-state imaging devices.
Described next are the details of the second problem. The amount of signals to be used is determined depending on the saturation charge level of a pinned photodiode which is used for a typical solid-state imaging device. One of advantages of pinned photodiodes is that a pinned photodiode can achieve almost complete transfer of the signal charges accumulated therein to a charge detecting unit adjacent thereto (complete transfer). Consequently, very little noise occurs in charge transfer, and this feature allows the pinned photodiodes to be widely used for solid-state imaging devices. The pinned photodiodes, however, cannot have a large capacity per unit area per photodiode. Hence, making a pixel fine causes a problem of a decrease in saturation charge level. A compact digital camera requires saturation electrons of 10,000 per pixel. When the size of a pixel is approximately as small as 1.4 μm, the maximum number of the saturation electrons is not more than 5,000. Today, the decrease in the number of saturation electrons when an image is generated is addressed, using noise suppression by a digital signal processing technique; however, it is difficult for the technique to obtain a naturally reproduced image. Furthermore, a high-end single-lens reflex camera is said to require saturated electrons of approximately 30,000 per pixel.
It is noted that, for MOS image sensors including crystalline silicon substrates, one of the structures to be considered for the sensors is to send light to the rear surface of a substrate instead of the front surface which is planed and has a pixel circuit formed thereon. Unfortunately, the structure merely prevents wiring for the pixel circuit from blocking the incident light, and cannot solve the first and second problems.
A promising solution to the two problems is a multi-layer solid-state imaging device (See Patent Literature 1, for example). The multi-layer solid-state imaging device includes a semiconductor substrate on which a pixel circuit is formed, an insulating film provided on the substrate, and a photoelectric converting film formed on the insulating film. Hence, the photoelectric converting film may be made of a material having a large optical absorption coefficient, such as amorphous silicon. For example, amorphous silicon of approximately 0.4 nm in thickness can absorb most of green light having a wavelength of 550 nm. Moreover, the multi-layer solid-state imaging device does not include a pinned photo diode, which makes it possible for a photoelectric converting unit in the device to have a larger the capacity and a higher saturation charge level. Furthermore, the multi-layer solid-state imaging device does not completely transfer the charges, and extra capacity is allowed to be actively added. Hence, sufficiently enough capacity is implemented for a fine pixel, which successfully solves the second problem. The multi-layer solid-state imaging device may be formed in a structure of a stack cell in a dynamic random access memory.